The organic light emitting diode (OLED) is one of the hotspots in the research field of panel display nowadays. As a current mode light emitting device, it has been more and more applied in high performance displays in virtue of its characteristics of self-luminous, quick response, wide visual angle and being capable of being fabricated on a flexible substrate. At present, in displays such as the mobile phone, PDA, digital camera, the OLED has begun to replace the conventional liquid crystal display (LCD). The design of the pixel driving circuit is the core technology of the OLED display, and has important research significance. Different from the TFT (thin film transistor)-LCD which makes use of stable voltages to control the brightness, the OLED belongs to current driving, which needs stable currents to control lighting. Due to the process procedure and the device aging, in an original 2T1C driving circuit (including two thin film transistors and a capacitance), threshold voltages of respective pixel points for driving the TFT are non-uniform. Thus it results in variation of the current that flows through the OLED of each pixel point, which renders the display brightness non-uniform, thereby influencing the display effect of the whole image.